Re: bgwriter_flush_after

On Thursday, October 14, 2021, Henry Francisco Garcia Cortez <
garcortez(at)gmail(dot)com> wrote:

> I was reading about bgwriter_flush_after I found this website
> https://postgreshelp.com/postgresql_shared_buffers/ however, I have some
> questions:
> First the parameter bgwriter_flush_after is amount of pages that operating
> system’s cache could download to disk every time Fsync is called.
> Second If I want to increase the parameter bgwriter_flush_after, for
> instance to 2MB Will I get a better performance? and
> Finally, What will be the cons of increasing this parameter?
>
>
Pushing cache data to disk consumes I/O bandwidth. If a page is updated it
eventually has to be pushed to disk. However, if it is updated again
before that mandatory flush only the final version needs to be written. By
attempting to spread out the I/O there is a decreased chance for that
efficiency to happen. Instead, the first update gets flushed well before
the mandatory time and then the second update also gets flushed. Thus the
total I/O consumed is double, though this is mitigated by the fact the time
period considered is spread out.

The benefits are circumstantial, which basically means figure out you have
a problem first then see whether this knob can help alleviate it. Though I
suppose there may be some useful guidelines to consider; I’m just not aware
of them.

David J.